Material processors-for reducing the particle size of bulk materials have been used for centuries in one form or another. One type of material processor of more recent use is known as a tub grinder. This type of apparatus includes a large tub having an open top for receiving bulk material to be processed. A stationary floor in the tub is in a generally horizontal plane.
A rotor member is mounted under the floor, and hammers or blades on the rotor extend into the tub through an opening in the stationary floor. The rotor is rotated at a high rate of speed, and the tub is rotated slowly. The hammers or blades on the rotor strike the bulk material and grind it into smaller particles. A screen having small apertures in it may be positioned in close proximity to the rotor under the floor. The bulk material cannot pass through the screen until the particles are smaller than the apertures.
As the tub is rotated the bulk material is continually urged against the revolving rotor member. The processed bulk material either falls through the screen system and is carried out by conveying or it is carried around the rotor by the hammers and thrown out the back side of the rotor and back into the tub where it is pushed around again by the rotating tub. Then the process is repeated.
Because the bulk material particles are typically forced through a screen in a conventional tub grinder, the speed of processing the bulk material is limited, and significant horsepower is required to operate the grinder. Bulk material stored outside tends to become wet and tough due to inclement weather. When putting such types of bulk material in the conventional tub grinder, the screen plugs up easily, thereby making grinding very difficult or impossible. Also, different screens must be used to obtain different particle size processed material. Another disadvantage of conventional tub grinders is that they tend to blow material out of the top of the tub when only a small amount of material is in the tub.
A conventional tub grinder is described, for example, in U.S. Pat. No. 4,106,706 (Burrows) which is referred to as a "cut-and-throw" machine where the rotating hammers move the bulk material down through the opening in the floor, past shear plates where the material is ground, and then the material is propelled at high speed out through an exit spout. The rapidly rotating hammermill generates an air stream which is intended to push the particles upwardly through the discharge chute.
The discharge chute or spout can easily become plugged when the bulk material is wet or is of the type which becomes easily compacted after being ground. Then the grinding operation must be stopped in order to manually clean out the discharge chute. This can be a very cumbersome and time-consuming task.
Also, the conventional tub grinder tends to cause undesirable separation of the ground material as it is blown or thrown out of the discharge chute. Not only does this result in the formation of a considerable amount of dust, it can also result in a very non-uniform distribution of the particles in the feed produced. Further, the conventional tub grinder of the type described above does not have the capability to allow a screen to be included adjacent the rotating hammers for the purpose of causing the particles to be reduced to even smaller size before they exit the machine.
European Patent Application No. 0121751 describes a tub machine having a rotor in the floor. The rotor includes knives which cut through bulk material such as hay or straw. The machine does not appear to be capable of handling material such as wood pallets or other material which is difficult to grind. The cut material falls downwardly to an auger for conveying the material to a blower for lifting and blowing the material out through a discharge chute. Thus, the apparatus can exhibit the same problems as exhibited by the Burrows machine described above.
Another conventional tub grinder is described in U.S. Pat. No. 4,003,502 (Barcell). This grinder has positionable blades on the sidewalls. The blades are rigid and are securely held in one position. Bolts extend through the blades and through the rib of the side wall to prevent movement of the blades while operating the grinder. The position of the blades is adjustable only by removing bolts when the grinder is at rest.
U.S. Pat. No. 4,087,051 (Moeller) also describes a tub grinder of the cut-and-throw type. S.U. 650,554 also describes a conventional tub grinder which has been modified to include an auger above the floor to carry the bulk material toward the center of the hopper to load the hammers evenly. Germany Patent 627,320 appears to describe a hammermill where the hammers are tilted away from vertical.
Another disadvantage of conventional tub grinders is that they tend to throw material upwardly out of the tub when the tub is nearly empty. This can be potentially dangerous when the material thrown out of the tub is a chunk of metal, rock, or other hard object.
Yet another problem with conventional tub grinders is that the rotating hammers sometimes pull large wads or slugs of bulk material from the tub area into the rotor area below the floor. When a wad or slug is pulled into the rotor this tends to slow the speed of the rotor and can result in nonuniform grinding of the material, thereby reducing the efficiency of the machine and increasing wear.
My prior U.S. Pat. No. 5,207,391 describes an improved tub grinder which overcomes many of the problems noted above.